Prescribing for Astigmatism

Déjà View: Trending Big

By Sara Bierwerth, OD, & Timothy B. Edrington, OD, MS, FAAO

Edouard Kalt and A. Eugene Fick in the late 1880s reported on the wearing of glass scleral contact lenses for correcting keratoconus. Lack of oxygen to the cornea limited the success of glass scleral and corneal lenses.

In the 1940s, Kevin Tuohy introduced corneal contact lenses made of plastic and ranging in overall diameter (OAD) from 10.8mm to 12.5mm. Wilhelm Sohnges and others produced “micro-corneal” lenses in the early 1950s that provided low amounts of oxygen to the cornea through tear exchange and because the OAD did not cover the entire corneal surface.

With the introduction of silicone-acrylate GP contact lenses in the late 1970s and fluorinated silicone acrylates in the 1980s, practitioners began prescribing corneal lenses with larger OADs. The Polycon II lens, which was available in several diameters, was often prescribed using a 9.5mm or 10.0mm OAD.

Enough With the History

Today we prescribe a variety of different OADs for our GP contact lens wearers. The optimal diameter is dictated by each patient's corneal toricity, corneal topography, or corneal condition. We often change the diameter to solve a problem or to optimize the lens-to-cornea fitting relationship. Smaller-diameter lenses minimize lens mass, which also increases oxygen transmissibility. Large-diameter corneal lenses tend to help with lens centration and allow for the incorporation of larger optic zone diameters to reduce patient symptoms of flare and glare. Larger-diameter lenses might also improve lens comfort if the superior edge of the lens is positioned beneath the eyelid.

Small, Medium, and Large

We still prescribe small (<9.0mm) OAD GP lenses for many of our keratoconus patients. Our “cosmetic” GP lens patients are generally prescribed lenses with OADs in the 9.5mm neighborhood. This includes the significant number of highly astigmatic patients who benefit from the crisp vision afforded by GP spherical and bitoric lenses. Many of our postsurgical patients benefit from wearing large corneal lenses with diameters ranging from 10.5mm to 11.5mm. This includes the majority of our post-radial keratotomy and post-penetrating keratoplasty patients. If their corneas are oblate, reverse geometry “intralimbal” designs are generally indicated.

Extra Large

Scleral GP lenses have been a welcome and impactful addition to our GP toolbox. Sclerals represent approximately 50 percent of our new irregular cornea fits. For keratoconus patients we still start with small-diameter corneal lenses. But if there is inadequate lens centration, frequent lens ejection, or lens intolerance after a reasonable adaptation period, we tend to prescribe a scleral design. For pellucid marginal degeneration patients, we first trial fit an intralimbal (~11.0mm OAD) design. But if the lens crosses the inferior limbus (almost always), we go directly to a scleral lens design. We also start our post-intrastromal corneal ring patients with sclerals.

It is 2012

Automated lathes, higher-oxygen-permeable GP materials, and increased lens design options have provided more, and better, choices for managing patients who benefit from GP lens wear. This has made prescribing and managing our GP patients more successful and rewarding. CLS

Dr. Bierwerth received her optometry degree from the Pennsylvania College of Optometry at Salus University. She is currently a cornea and contact lens resident at the Southern California College of Optometry. Dr. Edrington is a professor at the Southern California College of Optometry. You can reach him at tedrington@scco.edu.